Resistor coated with ceramic composition



May 23, 1950 s. GOODMAN RESISTOR COATED WITH CERAMIC COMPOSITION Filed D96. 12, 1946 lNVENTO/Q G. GOODMAN ATTORNEY Patented m za, sm

RESISTOR COATED WITH CERAMIC COMPOSITION Gilbert Goodman, Bayonne, N. 1., assignor a Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 12, 3946, Serial No. 715,771

2 Claims.

This invention relates to wire wound resistors coated with ceramic compositions.

In the manufacture or wire wound resistors utilizing resistance wire wound on cores made of insulating materials such as ceramics, it has been the practice to insulate the resistor with a ceramic coating. It has been found that the wires of such resistors sometimes become displaced from their positions when the resistor is fired. It is believed that this is caused by the fact that an enamel which is viscous and has a relatively high surface tension, tends to pull away from a break made in its surface by escaping gas bubbles which are formed from entrapped air and from the decomposition of certain constituents of the enamel when the latter is fired. As the break in the sur face or the enamel grows in size, the wires under the enamel coating may be pulled away from their positions, leaving the core of the resistor bare.

In accordance with this invention, this difdculty is overcome by adding a bismuth compound to a frit to produce an enamel having a high fluidity and a relatively low surface tension at the temperatures ordinarily used in firing enamels. During the firin of such an enamel an escaping gas bubble may break through the surface of the enamel, but the break closes over at once on account of the high fluidity and low surface tension of the enamel.

suitable for coating wire-wound resistors and for other applications may be made up of the following ingredients in parts by weight as indicated:

Parts by Weight Ingredient It is preferable to add the bismuth compound in such amounts that the final composition after w firing will contain from 17 per cent to 19 per cent g5 bismuth oxide. Throughout the present specifi- The addition of bismuth compounds has been I found particularly suitable for improving the properties of bore-silicate irits containing silica, boron oxide, and lead oxide. If desired such frits may contain various other non-essential oonstit uents to improve the properties.

Such ceramic compositions are particularly eitective for coatin wire wound resistors as shown in the drawing wherein:

Fig. 1 is an elevation of a wire wonndresistor v coated with a ceramic composition in accordance with the present invention, with a portionofthe ceramic coating composition cut away; and

Pig. 2 is a cross-section oi the wire-wouhd re stator 01 Fig. 1 taken along the line-L4.

It has been found that a lead-boro-silioate irit cation bismuth oxide is calculated as B1203. Aside from the bismuth compound, the percentages of the other ingredients in the frits shown above are not critical, and considerable departures may be made from th proportions shown above.

Although the irits A and B shown above are the preferred compositions, the frits C, D, E and ii shown below may also have a bismuth compound incorporated therein in accordance with the present invention. The bismuth compound may be added to irits C, D, E, F in such quanti-.

' ties that after firing, the composition will contain from 10 parts to 30 parts of bismuth oxide by weight.

Bismuth may be added to the frit compositions A, B, C, D, E, F, and to other trits, in the form of bismuth oxide or any compound which will form bismuth oxide on heating, such as bismuth hydroxide, bismuth subcarbonate, and bismuth subnitrate.

Parts by weight Considerable departures may be made in the proportions of the ingredients shown in frits A through F. For example, the red lead content of the composition may var widely. The final composition should contain the oxides of boron, lead and silicon, and no other ingredients than these are essential to the frits to which the bismuth compound may be advantageously added. However, other ingredients, such as those shown in the frits A through F, may be added to the mixture of flint, red lead and boric acid to improve the final properties of the frit.

For example, the oxides of nickel, cobalt and chromium may be added to reduce the solvent action of the molten frit on the wire of the resistor. The salts of sodium and potassium, and potash feldspar produce oxides of the alkali metals which act as fluxes. The oxides of calcium and titanium help to make the frit more resistant to chemical attack. The borax supplies sodium oxide, which acts as a fiux, as well as the essential boron oxide. Compositions containing a large amount of borax may contain a smaller amount of boric acid. Manganese dioxide may be added to color the frit.

It is to be understood that specific minerals mentioned above in the frit compositions are used as convenient sources of the oxides of which they are composed and that any other equivalent source which will yield a frit of the desired oxide composition may be employed.

The mixed frit composition is heated and when melted, it is allowed to flow into relatively cold water, thus "shattering" the frit. The frit is further pulverized in a ball mill.

The frit is suitable for use after the step of shattering and pulverization described above. However, the properties of the frit may be improved by incorporating it in the pulverized form in an enamel slip to form a composition which may have the following proportions by weight:

These components may be mixed in a ball mill with a suitable amount of water to provide a fluidity and viscosity consistent with the planned use of the enamel and with the available apparatus. Departures may be made in the proportions shown in the example of the slip. The zirconium oxide is added to provide opacity, while the addition of colloidal clay or of electrolytes such as strontium chloride or magnesium oxide provide thixotropic properties.

The slip may be used as a coating for a wirewound resistor as shown in Fig. 1 in which a core ill of insulating material is wound with electrical resistance wire ii, and provided with terminal bands l2 of any suitable type. The wire-wound resistor may be coated by dipping it in the slip or by allowing the slip to flow onto the resistor thereby covering it and forming a smooth contin uous coating H on the surface of the wire-wound resistor. The portion of the terminal bands I! which actually surround the core l0 may be coated with the slip as shown in Fig. 1, or they may be left uncoated if desired. The covered coated resistor is then dried in a normal atmosphere and subjected to heat to fire the enamel. The temperature of firing is ordinarily in the range of from about 500 degrees centigrade to 800 degrees centigrade, or above, and the time of heating should be at least long enough to cause the material to fuse. It will be found that during the firing bubbles escape easily from the surface of the enamel. The present compositions may be used for other purposes besides coating resistors.

Various changes and modifications may be made in the disclosed embodiments without departing from the spirit and scope of the invention.

What is claimed is:

1. An electrical resistor comprising a wirewound ceramic core coated with a fused enamel glaze resulting from the fusing of a mixture of colloidal clay and a finely divided lead borosilicate frit consisting of 25 per cent to 36.9 per cent $102, per cent to 24.5 per cent Pb304, 8.9 per cent to 30.9 per cent H330: and 10 per cent to 30 per cent B1203.

2. An electrical resistor comprising a wirewound ceramic core coated with a fused enamel glaze resulting from the fusing of a mixture containing 2 parts by weight of colloidal clay and 100 parts by weight of a' finely divided lead borosilicate frit consisting of per cent to 36.9 per cent B102, 10 per cent to 24.5 per cent Pb3O4, 8.9 per cent to 30.9 per cent H3180: and 10 per cent to per cent B1201.

GHrBERT GOODMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 257,643 Benas et a1 May 9, 1882 2,018,600 Brown Oct. 22, 1935 2,207,723 Deyrup July 16, 1940 2,370,448 Biefeld Feb. 27, 1945 2,385,580 Knox Sept. 25, 1945 2,425,032 Deyrup Aug. 5, 1947 

1. AN ELECTRICAL RESISTER COMPRISING A WIREWOUND CERAMIC CORE COATED WITH A FUSED ENAMEL GLAZE RESULTING FROM THE FUSING OF A MIXTURE OF COLLOIDAL CLAY AND A FINELY DIVIDED LEAD BOROSILICATE FRIT CONSISTING OF 25 PER CENT TO 36.9 PER CENT SIO2, 10 PER CENT TO 24.5 PER CENT PB3O4, 8.9 PER CENT TO 30.9 PER CENT H3BC3 AND 10 PER CENT TO 30 PER CENT BI2O3. 